[JoePAz]

New Geometry, sizing, and techy DH riding

So I have been riding since late 90's on various bikes. Over that time it has been mostly XC style bikes, but I did have one 7" bike in the mix

So here is me
5'7.5" Tall
inseam is longer 34" (measured from crotch to heel)
Weight 165lbs ish

Current bikes (taking my SS out of mix for simplicity)

2018 Epic Med
ETT = 23.4 - 595
Reach = 17.0 - 434
Stem = 80mm
Seat Angle 74.8
HA 69.5
Stack 23.5 - 597
100mm fork
Bars lower than saddle

I feel like this bike is well dialed in. Comfortable for long rides good on climbing and excellent cornering. I feel like new XC geo is working well and I understand it.

2013 5010 Large
ETT = 24.0 - 610
Reach = 16.8 - 426
Stem = 50mm
Seat Angle 73
HA68
Stack 23.6 - 599
130mm fork
bars even with saddle (saddle at pedal optimized height same as my epic, but with 125 mm dropper)

This where things get strange. Older Geometry 5010. I should be on a medium given my height, but this large works better. Seated it actually feels shorter and cramped a bit compared to me Epic. Could feel better with a 60mm stem instead of the 50mm. Reach is actually shorter than my Epic, but feels ok standing.Any tech DH features are approached standing so it is reach that is key. I like having the short 50mm stem on my 5010 and I know at least for me DH confidence is based on where my head is relative to the front wheel. The farther in front of my head the front wheel is the more perception of stability I have on the steeps and less feeling I have of going OTB. What does this all mean? I don't know it seems like I can get away with a longer bike for gnar at least on older geometry.

Now with the latest bikes most have 40mm stem. I fell like with a shorter than 24mm top tube the bike will feel short when seated an cramped, but the reach is getting longer.

Medium Offering
ETT = 24.0 - 611
Reach = 18.0 - 457
Stem = 40
Seat Angle = 76
HA 66.2
Stack 24.6 - 624

Medium Rimpo
ETT = 23.8 - 605
Reach = 17.6 - 447
Stem = 40
Seat angle 76
HA65.9
Stack 24.5 - 622

So wonder what really going on. For climbing the steeper seat tube angles help get weight forward when standing to keep the front end down. I had for a time 7" long travel 26er. It was a small frame and one size too small for me, but was interesting. Climbed like crap with front end wandering around, but down hill was very good. Not so much in highspeed stuff, but really in low speed tech. I felt very confident in getting through steep stuff and rock features. I believe this was due to where front wheel was. I had 50mm stem on that bike with really short ETT and reach, but with 67deg HA and 180mm fork that front tire was way out in front. Bad for climbing, but great for picking through nasty tech. I had the 5010 at the same time it was so much better at speed due to longer wheelbase I think.

Now all this gets me thinking. What is the right feel for bike destined for steeps and DH tech? I know what a good XC bike feels like for fit, but I am not sure I have feeling of fit dialed in for new geo techy bike. Part of me thinks I need to feel like my XC bike seated, but that probably is not idea. Heck I need like 24.5" (622mm) ETT wih a 40/50 stem for the bike to feel like my Epic when seated. Remember my epic has 74.8 Deg seat angle so not far off the new 76 deg stuff. Of course with a 24.5" (622mm) ETT the reach is out past 18" (457mm) which is longer than am used to, but if I measure from cranks to where the bars are on the short 40mm stems is less than on my epic. I guess I need to sit more upright on these techy bikes, but I wonder how much I need to adapt vs making the bike fit.

Now I can hop on a bike and feel it in the parking lot or even have a bike fit done, but most of these fits are around seated pedaling. That I have a solid feel for (even if not perfect, enough to be close), but not so much for what I need for DH stability and agilty in tech features. I am less interested in DH speed or jumping that riding gnar at lower speeds.

So just some thoughts.

 

[mack_turtle]

I can't make head or tails of those numbers in inches. If you clarify it with millimeters, I'll have a frame of reference.

These days, I look at reach first, as that can't be adjusted much. I know any bike I'll want to ride will have a 45-55mm stem, but I can move the saddle position and handlebar height around quite a bit, while the reach remains almost completely static.

 

[JoePAz]

I can't make head or tails of those numbers in inches. If you clarify it with millimeters, I'll have a frame of reference.

These days, I look at reach first, as that can't be adjusted much. I know any bike I'll want to ride will have a 45-55mm stem, but I can move the saddle position and handlebar height around quite a bit, while the reach remains almost completely static.

Ok so you can't adjust reach, but you can play with stems

my Epic is 20.2(513mm) from cranks to where my bars are (reach+ stem). That feels good

My 5010 is 18.7 (476mm) from cranks to where my bars are (reach+ stem). That feels good.

But it is in effect so much shorter. But given where the front wheel is due to HA and travel on the fork the front axle is over 2" farther forward (57mm)

So what really matters? For XC seated position is very critical since you need to get power down effectively and you work around the rest.

 

[mack_turtle]

I don't play with stems much. The stem is for handling, not fitting. Less than 1 cm of adjustment there. You can make a bike that is too small "fit" by putting a long stem on it, but the handling goes to crap if you do. That's why I say reach is effectively static on the frame.

When you measure cranks-to-handlebar, is that to a midpoint between the grips, or where the stem clamps the bar? That ignores handlebar width and sweep, as well as the rise and rotation, which can vary the end result by a lot.


Seated position is important, but not if it compromises handling over rough terrain. The two types of fitting need to strike a balance between the manueverability of a trials bike and the pedaling comfort of a touring bike.

That might not be helpful, but it's a counter to the absurd amount of importance people put on how a bike fits when you're sitting and spinning the cranks. That is useful only of the bike is going to stay mounted to a trainer.

 

[JoePAz]

...
When you measure cranks-to-handlebar, is that to a midpoint between the grips, or where the stem clamps the bar? That ignores handlebar width and sweep, as well as the rise and rotation, which can vary the end result by a lot....

The numbers are based on reach + stem length so really to the center of the bars. Yeah bar sweep and width makes a difference, but I am ignoring that. Reach and stem length are easy to obtain. distance to the grip is much harder and lots of geometry is involved and that to my mind begins to over think things. Once you buy a bike swapping stems and bars is not very hard. As you state reach is built in.

Bike fits are usefull for XC bikes since the goal (just like in road bikes) is to get lots of power to the group comfortably and effectively. You do make some compromises for fit on a mtn bike because of the terrain, but you start from the same point. You simply adapt to the needs to go fast down and tweak your set-up to maintain as much power as possible

DH riding (I say DH only to imply standing and less pedaling, not actual DH racing) is focused standing are more dynamic position. Here seated riding may need some adaptation, but with use of dropper posts less so than years ago.

I guess I am trying to understand how reach is so critical. On paper my Epic has longer reach than my 5010 even without the stem. Once the stem is added in my Epic is much "longer". Makes me wonder if I should get a big bike with the same (or close it) reach+ stem that I am used to with my Epic. If so that puts me closer to some "Large" bikes rather the mediums. That feel odd given the longer reaches on the newer bikes anyway.

BTW.. Large Ibis Ripmo
ETT = 24.9 - 632
Reach = 18.6 = 472
Stem 40mm
Reach + Stem = 20.2 - 512mm or same as my Medium Epic with that big 80mm stem.

But now the ETT is super long, but strangle my seat to the bars calucuation is the same as my Epic also 26.6 - 675 vs L Rimpo at 26.5 - 672mm Yeah both are calculated, but it makes you wonder. I guess may be I should be "sitting up" more on these mid-long travel bikes.

 

[Harold]

Using a Santa Cruz for comparison kinda screws things up. I'm 5'8 and large santa cruzes just fit me better. except my inseam is shorter than yours, and so I end up having to use the dropper just to set the seat height at a comfortable point for pedaling.

I ride mediums in everything else. I could definitely see with your longer torso, you might be feeling better on bikes that are even longer than what I like.

Reach, as a dimension, is a frame-only measurement that'll consider your riding position when standing. So, particularly, for those downhill coasting scenarios. It's not so useful for seated pedaling positions, where you are looking at the saddle to bars distance.

I don't remember the specifics at this point, but of my 2 mtb's, my Bucksaw has the shorter reach. But it has a slightly longer stem and a slacker seat tube angle, and so the saddle to bars distance is maybe 10mm longer than my Pedalhead. It's comfy. I've done 50mi rides on it. The Pedalhead is new enough that I don't have the fit adjustments dialed in just yet for seated pedaling. I need to fiddle with it some more still. But even now, it's much easier for me to maneuver the bike in technical terrain, even though it's a longer bike overall and is very nearly as heavy. It's also more capable on downhills.

 

[Legbacon]

You can't compare reach without looking at stack since reach is measured at the top of the head tube. I don't think that an AM bike should duplicate the fit of a XC race bike. The bars will probably be higher and closer even though the reach measurement is longer. I find that longer reach/front centre combined with a 66-65° HTA helps immensely going down steep chunky trails. What constitutes a medium is all over the place. I am good with large on most brands but medium on more progressive bikes. If you came to the shop I would give you a medium Knolly Fugitive to demo first. I'm 5'9" on a large with a 31" inseam on a large, slammed 40mm stem, 20mm rise 8° sweep bar, and saddle as forward as the markings allow. Best technical climbing bike I can imagine and makes me look competent going down.

 

[jeremy3220]

You can't compare reach without looking at stack since reach is measured at the top of the head tube.

Yep, reach is always relative to where the measurement was taken vertically along the steering axis (stack).

There's a couple interviews I saw recently, one with Greg Minnaar (interviewed by Cathro) and one with Neko Mullaly (by Worldwide Cyclery). They both talked about the length progression of DH bikes. Greg talked about how a few years ago his current bike would have probably felt too long and he needed the progression of bikes getting longer over time to feel comfortable on a bike that long. Neko talked about how you'd think a smaller bike would be easier to maneuver around but it's actually more work because on a longer bike it's easier to stay centered. Which I can confirm having recently ridden a Bronson that was too small, I was surprised how much I was having to move back and forth to balance the bike and myself.

I think this is where reach+stack are critical...if your bars are too close to your feet you'll be less stable and have to do more work to compensate for that (stabilize the system). Imagine how stable a 3 ft tall table with the legs 1 ft apart is vs a 3 ft tall table with the legs 5 ft apart. You should be able to get into a flat back 'attack' position and still feel centered. If you end up with a really closed hip angle and your butt sticking out behind the bike the reach isn't long enough. On the trail you'll end up hunched and have to move around a lot to keep your weight centered. I think this is most easily felt transitioning from braking to cornering. On a bike that's too short you'll want to get really far back while braking then struggle to get into cornering position and lean the bike as you transition off the brakes. In the corner you'll feel like you're never in quite the right position. On a properly fitting bike you shouldn't have to move forwards and backwards much and it will feel more intuitive.

 

[DIRTJUNKIE]

New Geomerty, sizing, and techy DH riding.

^Joe, can you please spell check and edit the title of the thread before Finch loses it again.

 

[JoePAz]

Ok so my Epic and 5010 have pretty much the same stack 23.5 (597) and 23.6 (599) The Offering and Ripmo are 1" taller stack 24.6 (624) and 24.5 (622). I run about 1" of spacers under stem on both Epic and 5010 (5010 has slight rise to the bars, epic is flat). So both Epic and 5010 reach are comparable since the stack's are so close. However the other 2 are taller.

Now I can see how stack impacts ETT since ETT is measured from the top tube to the a fictional intersection of the seat tube parallel to the ground. Since the seat tube is always at an angle the taller the stack the longer the ETT. However for reach you are measuring from the BB horizontal to the heat tube. I don't see how a taller or shorter stack will change that measurement. I do see how stack height can impact feel as taller stacks raise the bars some of which can be compensated for with spacers/riser bars/positive or negative stems. If you assume the same spacer stack, bars and stem then yes taller or shorter stack will impact how you feel when standing. I also can see where a tall or short stack can cause fit issues that you have hard time correcting with stem,spacers and bars.

You should be able to get into a flat back 'attack' position and still feel centered. If you end up with a really closed hip angle and your butt sticking out behind the bike the reach isn't long enough. On the trail you'll end up hunched and have to move around a lot to keep your weight centered.

Hmm thinking about this.

 

[jeremy3220]

For a given reach measurement, the greater the stack the further away the steering axis is from the BB. In other words, a bike with the same reach but lower stack is actually shorter.

 

[Harold]

However for reach you are measuring from the BB horizontal to the heat tube.

There's some virtual line drawing going on. Horizontal line from the TOP of the head tube. Vertical line from the bb.

It's making a triangle. Jeremy is specifically referring to the hypotenuse of the triangle made from the reach and the stack dimensions.

 

[JoePAz]

For a given reach measurement, the greater the stack the further away the steering axis is from the BB. In other words, a bike with the same reach but lower stack is actually shorter.

The straight line distance (not parallel to the ground) is clearly longer as stack height goes up. A^2+ B^2= C^2 type stuff, but how is that critical? Things like head angle and fork AC and trail determine the actual location of front wheel.

 

[mack_turtle]

You can't compare reach without looking at stack since reach is measured at the top of the head tube.

This is valid. If you are comparing a few bikes that have the same length fork and similar head tube lengths, which is likely the case if you've narrowed down a few similar bikes that you might buy, and you know that you're going to use a consistent handlebar size and stem length, then you can effectively use reach alone to compare bikes. This has potential to leave a lot out, so it's still a good idea to factor in stack with reach.

However, I have been advocating for a long time that we can simplify this quite a bit if we start comparing what I have been calling effective down tube. The stack and reach form a right triangle. The EDT is the hypotenuse of that triangle. You can use this measurement to compare the effective distance from the BB to the top of the head tube, which, assuming you don't stray far from your preferred crank arm length, pedal thickness, and handlebar/ stem combo, should give you a more direct comparison of how two bike geometries compare when it comes to standing and wrangling the bike over terrain, especially when going down hill. This is how Sam Hill reportedly sets up his bike.

Once you have EDT for a few similar bikes, you can tell if the bike is going to allow a compromise between swallowing you up between the wheels with such stability that you can't steer the thing and teeter-tottering on top of the bike in an unstable fashion. You will learn how much wiggle room stem and handlebar positioning will allow from this point.

Of course, then you have to take seat tube angle and all that into consideration for seated pedalling efficiency, but that's a separate but interconnected topic.

 

[jeremy3220]

The straight line distance (not parallel to the ground) is clearly longer as stack height goes up. A^2+ B^2= C^2 type stuff, but how is that critical? Things like head angle and fork AC and trail determine the actual location of front wheel.

So the horizontal lines are reach. The green 'bike' has a higher stack. They both have the same reach. The diagonal lines are the steering axes of each bike. Both have the same HTA. For a given reach the higher the stack the further away the steering axis is (along with the front wheel).

 

[JoePAz]

So the horizontal lines are reach. The green 'bike' has a higher stack. They both have the same reach. The diagonal lines are the steering axes of each bike. Both have the same HTA. For a given reach the higher the stack the further away the steering axis is (along with the front wheel).

View attachment 1255411

The difference between green and blue lines will be shown in Front center measurement. Front center. (Distance from BB to axle on the front wheel) will impact handling, but not so much fit. Also this implies that green bike has longer axle to crown however the actual actual to crown will also have to take into account head tube length. So in the above drawing the green line would have a longer fork vs the blue line if we assume same head tube length.

 

[Ryder1]

So the horizontal lines are reach. The green 'bike' has a higher stack. They both have the same reach. The diagonal lines are the steering axes of each bike. Both have the same HTA. For a given reach the higher the stack the further away the steering axis is (along with the front wheel).

View attachment 1255411

Yes. I personally account for this stack delta by subtracting/adding 4mm from Reach for every 10mm of Stack delta, assuming a 67* HTA. For a 72* HTA, it's 3mm of Reach for every 10mm of Stack delta. This allows me to compare Reach of various bikes in my head.

 

[jeremy3220]

The difference between green and blue lines will be shown in Front center measurement. Front center. (Distance from BB to axle on the front wheel) will impact handling, but not so much fit. Also this implies that green bike has longer axle to crown however the actual actual to crown will also have to take into account head tube length. So in the above drawing the green line would have a longer fork vs the blue line if we assume same head tube length.

It doesn't matter how that stack height is obtained (head tube length, bigger wheel, etc). If the reach is the same and the stack height different, the steering axis is in a different location which affects fit. If you put your stem at the same height on the blue and green bike, on the blue bike it will closer (shorter effective reach). You could run a longer stem but then you have a long stem.

 

[JoePAz]

It doesn't matter how that stack height is obtained (head tube length, bigger wheel, etc). If the reach is the same and the stack height different, the steering axis is in a different location which affects fit. If you put your stem at the same height on the blue and green bike, on the blue bike it will closer (shorter effective reach). You could run a longer stem but then you have a long stem.

Yeah, but why put your bars higher or lower due to stack? The bar height should be set by fit not the other way around. That is what spacers and riser bars are for.
Of course with certain stack heights you may not be able to get the bars where you need them. So that must be considered. Now 5mm of reach difference is in fact pretty minimal.

 

[jeremy3220]

Yeah, but why put your bars higher or lower due to stack?

Idk, I didn't mention that. Spacers and riser bars move your hand position, they do not move the steering axis though. I can use spacers, long stems and riser bars on a bike two sizes too small and get my hands where I would on the proper size bike but obviously it will handle like crap. The steering axis distance from the BB is really what gives a bike it's length. For example a Giant Reign has a reach of about 19.5" which is about the reach of my hardtail. However a Giant Reign is too small for me because that measurement is taken at a low stack height. I could add riser bars but then my hands are further forward of the steering axis which is the same thing as using a bunch of spacers and a long stem. IOW, if I buy an enduro bike I want my hands close to the steering axis for certain handling characteristics so that steering axis needs to be far enough away, I can't just look at the reach numbers.